Generally in inkjet printing, ink is supplied in four different colors in the “CMYK” scheme. These colors are cyan, magenta, yellow and black. Cyan, magenta and yellow are the subtractive primary colors, rather than the additive primary colors of red, green and blue. This means that colors must be subtracted in order to obtain white (the color of the paper), rather than adding colors to obtain white.
It is possible to create black from a combination of the three subtractive colors, by removing all red, blue and green. However, this results in high use of all three color inks, and so the fourth color, black, is added to print cartridges to reduce use of the other three colors.
In general, during home printing, the consumption of black ink is higher than the consumption of the other colors. Therefore, the black ink supply will generally run out first. Once one of the ink colors has run out, generally the black ink supply, the ink cartridge (containing all ink colors) must be replaced. This is because, once one supply is exhausted, air will be drawn from the empty reservoir into the printhead. The printhead can be damaged if too much air is drawn through it. Therefore, when the black ink supply is low, before the supply runs out completely and the cartridge has to be replaced, the printer will enter “limphome” mode. In this mode, the printer does combine the cyan, magenta and yellow colors to produce a black color. In this way, the black ink supply is conserved and the life of the cartridge is extended until one of the color ink reservoirs is empty. However, even in limphome mode, the black ink reservoir of the cartridge is still functioning, and so air may still travel into the printhead when the black ink level is low.